U.S. patent number 7,589,928 [Application Number 11/724,739] was granted by the patent office on 2009-09-15 for magnetic recording device including a thermal proximity sensor.
This patent grant is currently assigned to Seagate Technology LLC. Invention is credited to Housan S. Dakroub, William Bruce Fitzpatrick, Kaizhong Gao, Insik Jin, Jeffrey Howard Lake, Sining Mao, Mallika Roy, Dadi Setiadi, Song S. Xue.
United States Patent |
7,589,928 |
Roy , et al. |
September 15, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetic recording device including a thermal proximity sensor
Abstract
A system includes a magnetic device for writing to and reading
from a magnetic medium and a sensor disposed adjacent to the
magnetic device and proximate to the magnetic medium. The sensor
generates signals related to thermal variations in the sensor
caused by changes in a distance between the magnetic device and the
magnetic medium.
Inventors: |
Roy; Mallika (Edina, MN),
Gao; Kaizhong (Eden Prairie, MN), Jin; Insik (Eagan,
MN), Fitzpatrick; William Bruce (Prior Lake, MN),
Dakroub; Housan S. (Dearborn Heights, MI), Mao; Sining
(Eden Prairie, MN), Xue; Song S. (Edina, MN), Lake;
Jeffrey Howard (Bloomington, MN), Setiadi; Dadi (Edina,
MN) |
Assignee: |
Seagate Technology LLC (Scotts
Valley, CA)
|
Family
ID: |
39762411 |
Appl.
No.: |
11/724,739 |
Filed: |
March 16, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080225426 A1 |
Sep 18, 2008 |
|
Current U.S.
Class: |
360/75;
360/69 |
Current CPC
Class: |
G11B
5/1278 (20130101); G11B 5/607 (20130101); G11B
5/6005 (20130101); G11B 5/40 (20130101); G11B
5/6064 (20130101); G11B 5/314 (20130101); G11B
5/3133 (20130101) |
Current International
Class: |
G11B
21/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Olson; Jason C
Attorney, Agent or Firm: Kinney & Lange PA
Claims
The invention claimed is:
1. A system comprising: a magnetic device comprising a main pole
for writing to a magnetic medium and a read element for reading
from the magnetic medium; a thermal sensor spaced from the main
pole and separate from the read sensor, disposed adjacent to the
magnetic device and at or near a medium confronting surface
proximate the magnetic medium, wherein the sensor generates signals
related to thermal variations caused by changes in a distance
between the magnetic device and the magnetic medium, wherein a
resistance of the sensor increases in response to the thermal
variations, and wherein the increase in resistance is measured by
passing a sense current through the sensor; a heating element for
heating the sensor to vary a sensitivity of the sensor to the
thermal variations; wherein the sense current passes through the
sensor and the heating element in series or in parallel; an
actuator separate from the sensor for varying the distance between
the magnetic device and the magnetic medium; and a controller that
controls the actuator to control the distance between the magnetic
device and the magnetic medium in response to the sensor
signals.
2. The system of claim 1, wherein the sense current passes though
the sensor and heating element in series.
3. The system of claim 1, wherein the sense current passes through
the sensor and heating element in parallel.
4. The system of claim 1, wherein the sensor further generates
signals related to a clearance of the magnetic device from the
magnetic medium.
5. The system of claim 1, wherein the sensor is spaced at least 1.0
.mu.m from a tip of the main pole.
6. The system of claim 1, wherein the sensor is comprised of a
material having a high thermal coefficient of resistivity and low
magnetoresistance.
7. The system of claim 1, wherein the sensor is comprised of an
inert material.
8. The system of claim 1, wherein the sensor is embedded in a
metallic material.
9. A magnetic recording system comprising: a magnetic recording
device comprising a main pole for writing data to a magnetic medium
and a read element for reading data from the magnetic medium, and
having a medium confronting surface facing the magnetic medium; a
thermal sensor spaced from the main pole and separate from the read
element, disposed adjacent to the magnetic recording device, at or
near the medium confronting surface and proximate to the magnetic
medium, wherein the sensor generates signals related to thermal
variations caused by changes in a distance between the magnetic
recording device and the magnetic medium, wherein a resistance of
the sensor increases in response to the thermal variations, and
wherein the increase in resistance is measured by passing a sense
current through the sensor; a heating element for heating the
sensor to vary a sensitivity of the sensor to the thermal
variations, wherein the sense current passes through the sensor and
heating element in series or in parallel; an actuator separate from
the sensor for varying the distance between the magnetic device and
the magnetic medium; and a controller that controls the actuator to
control the distance between the magnetic recording device and the
magnetic medium in response to the sensor signals.
10. The magnetic recording system of claim 9, wherein the
controller is operable to map a topography of the magnetic medium
based on the sensor signals.
11. The magnetic recording system of claim 9, wherein the sensor
further generates signals related to a thermal spike caused by
contact between the magnetic recording device and asperities on the
magnetic medium.
12. The magnetic recording system of claim 9, wherein the distance
between the magnetic recording device and the magnetic medium is
controlled by heating at least a portion of the magnetic recording
device.
13. The magnetic recording system of claim 9, wherein the sensor
further generates signals related to a clearance of the magnetic
device from the magnetic medium.
14. The magnetic recording system of claim 9, wherein the sense
current passes through the sensor and heating element in
series.
15. The magnetic recording system of claim 9, wherein the sense
current passes through the sensor and heating element in
parallel.
16. The magnetic recording system of claim 9, wherein the sensor is
spaced at least 1.0 .mu.m from a tip of the main pole.
17. The magnetic recording system of claim 9, wherein the sensor is
comprised of a material having a high thermal coefficient of
resistivity and low magnetoresistance.
18. The magnetic recording system of claim 9, wherein the sensor is
comprised of a noble metal.
19. A system comprising: a magnetic device comprising a main pole
for writing to a magnetic medium and a read element for reading
from the magnetic medium; a thermal sensor spaced from the main
pole and separate from the read element, disposed adjacent to the
magnetic device and at or near a medium confronting surface
proximate the magnetic medium, wherein the sensor generates signals
related to thermal variations caused by changes in a distance
between the magnetic device and the magnetic medium, wherein a
resistance of the sensor increases in response to the thermal
variations, wherein the increase in resistance is measured by
passing a sense current through the sensor, and wherein the sensor
is heated by increasing the amplitude of the sense current to
increase a sensitivity of the sensor to thermal variations; an
actuator separate from the sensor for varying the distance between
the magnetic device and the magnetic medium; and a controller that
controls the actuator to control the distance between the magnetic
device and the magnetic medium in response to the sensor
signals.
20. A magnetic recording system comprising: a magnetic recording
device comprising a main pole for writing data to a magnetic medium
and a read element for reading data from the magnetic medium, and
having a medium confronting surface facing the magnetic medium; a
thermal sensor spaced from the main pole and separate from the read
element, disposed adjacent to the magnetic recording device, at or
near the medium confronting surface and proximate to the magnetic
medium, wherein the sensor generates signals related to thermal
variations caused by changes in a distance between the magnetic
recording device and the magnetic medium, wherein a resistance of
the sensor increases in response to the thermal variations, wherein
the increase in resistance is measured by passing a sense current
through the sensor, and wherein the sensor is heated by increasing
the amplitude of the sense current to increase a sensitivity of the
sensor to thermal frictional heating; an actuator separate from the
sensor for varying the distance between the magnetic device and the
magnetic medium; and a controller that controls the actuator to
control the distance between the magnetic recording device and the
magnetic medium in response to the sensor signals.
Description
BACKGROUND OF THE INVENTION
The present invention relates to magnetic devices. More
particularly, the present invention relates to managing the
head-to-medium spacing (HMS) in a recording system using thermal
proximity measurement.
In magnetic recording systems, a rapid increase in the areal
density of magnetic media has led to reduction of the spacing
between the head and the medium down to less than 10 nm.
Maintaining a constant head-to-medium spacing (HMS) is important
throughout the life of the magnetic recording system, since the
close proximity of the head to the medium makes the drive
susceptible to reliability issues that could lead to temporary
modulation of the HMS. Most conventional recording systems do not
provide reliable approaches to monitoring the HMS in-situ.
The difference in temperature between the head and the medium
results in heat transfer during operation, which may be represented
by:
.times..DELTA..times..times..times..times. ##EQU00001## where q is
the amount of heat transferred between the head and the medium, h
is the separation between the head and the medium, p is the
pressure at the sensor, c is a constant that depends on the
molecular properties of the air surrounding the head and the
medium, T is the ambient temperature, K.sub.a is the conductivity
of air, and .DELTA.T is the difference in temperature between the
head and the medium. Thus, because the amount of heat transferred
between the head and the medium is proportional to .DELTA.T and
inversely proportional to h, the temperature at the medium
confronting surface of the head may be measured to continuously
monitor changes in the HMS.
Some conventional systems attempt to capitalize on this phenomenon
by monitoring temperature changes in the reader of the head.
However, in order to improve the detection sensitivity of the
reader, the reader element had to be biased at a relatively high
voltage to sense the change in resistance in the reader element
caused by the temperature change. This can lead to compromised
reader life and, because the reader is highly magnetoresistive, can
also make it difficult to differentiate between the thermally and
magnetically induced components of resistance change in the
reader.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a system including a magnetic
device for writing to and reading from a magnetic medium and a
sensor disposed adjacent to the magnetic device and proximate to
the magnetic medium. The sensor generates signals related to
thermal variations in the sensor caused by changes in separation
between the magnetic device and the magnetic medium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a transducing head and a
thermal proximity sensor disposed adjacent to the transducing
head.
FIG. 2 is a medium confronting surface view of a write element tip
and the thermal proximity sensor for monitoring the head-to-medium
spacing of the transducing head.
FIG. 3 is a graph of the difference in on-medium and off-medium
resistance of the thermal proximity sensor as a function of an
applied writer heater power.
FIG. 4 is a graph showing the effect on the sensor when the writer
heater power is increased until contact is made between the
transducing head and the magnetic medium.
FIGS. 5A-5D are schematic views of configurations for incorporating
a sensor heater with the thermal proximity sensor.
FIG. 6 is a graph of the response of the thermal proximity sensor
when it contacts an asperity on a magnetic medium.
FIG. 7 is a block diagram of a system for providing in-situ control
of the head-to-medium spacing based on signals provided by the
thermal proximity sensor.
DETAILED DESCRIPTION
FIG. 1 is a cross-sectional view of transducing head 10 including
thermal proximity sensor 12 to provide signals related to the
head-to-medium spacing (HMS) of transducing head 10. Sensor 12 will
be described in more detail with regard to FIGS. 2-6. Transducing
head 10 includes reader 14 and writer 16 that define medium
confronting surface 18. Reader 14 includes bottom shield structure
22, read element 24, read gap 26, and top shield structure 28.
Writer 16 includes first return pole 30, first magnetic stud 32,
main pole 34, second magnetic stud 36, second return pole 38, first
conductive coil 40, and second conductive coil 42. Main pole 34
includes main pole body 44, yoke 46, and main pole tip 48.
Reader 14 and writer 16 are each multi-layered devices, and writer
16 is stacked on reader 14 in a piggyback configuration in which
layers are not shared between the two elements. In other
embodiments not illustrated, reader 14 and writer 16 may be
arranged in a merged-head configuration (in which layers are shared
between the two elements) and/or reader 14 may be formed on writer
16.
Read gap 26 is defined on medium confronting surface 13 between
terminating ends of bottom shield 22 and top shield 28. Read
element 24 is positioned in read gap 26 adjacent medium confronting
surface 13. Read gap 26 insulates read element 24 from bottom
shield 22 and top shield 28. Read element 24 may be any variety of
different types of read elements, such as a tunneling
magnetoresistive (TMR) read element or a giant magnetoresistive
(GMR) read element. In operation, magnetic flux from a surface of
magnetic medium 60 causes rotation of a magnetization vector of
read element 24, which in turn causes a change in electrical
resistivity of read element 24. The change in resistivity of read
element 24 can be detected by passing a current through read
element 24 and measuring a voltage across read element 24. Shields
22 and 28, which may be made of a soft ferromagnetic material,
guide stray magnetic flux from medium layer 66 away from read
element 24 outside the area of medium layer 66 directly below read
element 24.
In writer 16, first return pole 30, second return pole 38, first
magnetic stud 32, and second magnetic stud 36 may comprise soft
magnetic materials, such as NiFe. Conductive coils 40 and 42 may
comprise a material with low electrical resistance, such as Cu.
Main pole body 44 may comprise a high moment soft magnetic
material, such as CoFe. Yoke 46 may comprise a soft magnetic
material, such as NiFe or CoNiFe, to improve the efficiency of flux
delivery to main pole body 34. First conductive coil 40 surrounds
first magnetic stud 32, which magnetically couples main pole 34 to
first return pole 30. Second conductive coil 42 surrounds second
magnetic stud 36, which magnetically couples main pole 34 to second
return pole 38. First conductive coil 40 passes through the gap
between first return pole 30 and main pole 34, and second
conductive coil 42 passes through the gap between main pole 34 and
second return pole 38.
Reader 14 and writer 16 are carried over the surface of magnetic
medium 60, which is moved relative to transducing head 10 as
indicated by arrow A such that main pole 34 leads first return pole
30, trails second return pole 38, and is used to physically write
data to magnetic medium 60. In order to write data to magnetic
medium 60, current is caused to flow through second conductive coil
42. The magnetomotive force in the coils causes magnetic flux to
travel from main pole tip 48 perpendicularly through medium layer
66, across SUL 64, and through second return pole 38 and first
magnetic stud 36 to provide a closed magnetic flux path. The
direction of the write field at the medium confronting surface of
main pole tip 48, which is related to the state of the data written
to magnetic medium 60, is controllable based on the direction that
the current flows through second conductive coil 30.
Stray magnetic fields from outside sources, such as a voice coil
motor associated with actuation of transducing head 10 relative to
magnetic medium 60, may enter SUL 64. Due to the closed magnetic
path between main pole 34 and second return pole 38, these stray
fields may be drawn into writer 16 by second return pole 38. In
order to reduce or eliminate these stray fields, first return pole
30 is connected to main pole 34 via first magnetic stud 32 to
provide a flux path for the stray magnetic fields. In addition, the
strength of the write field through main pole 34 (due to current
flowing through second conductive coil 42) may be augmented by
causing current to flow through first conductive coil 40. The
magnetomotive force in the coils causes magnetic flux to travel
from main pole tip 48 perpendicularly through medium layer 66,
across SUL 64, and through first return pole 30 and first magnetic
stud 32 to provide a closed magnetic flux path. The direction of
the current through first conductive coil 40 is opposite that of
the current through conductive coil 42 to generate magnetic flux in
the same direction through main pole 34. The effect of employing
two return poles and two conductive coils is an efficient driving
force to main pole 34, with a reduction on the net driving force on
first return pole 30 and second return pole 38.
Writer 16 is shown merely for purposes of illustrating a
construction that may be used in a transducing head 10 including
sensor 12, and variations on the design may be made. For example,
while main pole 34 includes main pole body 44 and yoke 46, main
pole 34 can also be comprised of a single layer of magnetic
material. In addition, a single trailing return pole may be
provided instead of the shown dual return pole writer
configuration. Also, a shield may additionally be formed to extend
from first return pole 30 toward main pole 34 proximate medium
confronting surface 13 in a "trailing shield" magnetic writer
design. Furthermore, writer 16 is configured for writing data
perpendicularly to magnetic medium 60, but writer 16 and magnetic
medium 60 may also be configured to write data longitudinally.
Transducing head 10 confronts magnetic medium 60 at an air bearing
surface (ABS). Magnetic medium 60 includes substrate 62, soft
underlayer (SUL) 64, and medium layer 66. SUL 64 is disposed
between substrate 62 and medium layer 66. Magnetic medium 60 is
positioned proximate to transducing head 10 such that the surface
of medium layer 66 opposite SUL 64 faces reader 14 and writer 16.
Magnetic medium 60 is shown merely for purposes of illustration,
and may be any type of medium that can be used in conjunction with
transducing head 10, such as composite media, continuous/granular
coupled (CGC) media, discrete track media, and bit-patterned
media.
As will be described herein, sensor 12 is disposed at or near
medium confronting surface 13 and provides signals related to
thermal variations in sensor 12 caused by changes in separation
d.sub.hms between transducing head 10 and magnetic medium 60.
Sensor 12 may be made of a material having a high thermal
coefficient of resistivity and low magnetoresistance at operating
temperatures of transducing head 10 such that the resistance of
sensor 12 is a function of its temperature. The change in
resistance may be detected by passing a sensor current I.sub.S
through sensor 12 and measuring the resulting voltage drop across
sensor 12.
FIG. 2 is a medium confronting surface view of main pole tip 48 and
sensor 12 separated by insulating layer 70. In addition, FIG. 2
shows sensor 12 embedded in protective embedding material 72.
Sensor current I.sub.S, which may be alternating current or direct
current, is delivered to sensor 12 by electrical contacts 72a and
72b, which are connected to a current source (not shown). Sensor
current I.sub.S passes through sensor 12 parallel to medium
confronting surface 13 and the trailing edge of main pole tip 48.
The voltage drop across sensor 12 may be measured and monitored to
detect changes in the HMS.
Insulating layer 70 is made of a material that prevents electrical
and magnetic interactions between sensor 12 and main pole 34.
Sensor 12 is placed proximate to main pole tip 48 to maximize the
sensitivity of sensor 12 to the HMS of writer 16. However, sensor
12 is spaced from main pole tip 48 by distance d.sub.sp, which is
an effective distance to prevent data erasure or interference with
the operation of writer 16. In some embodiments, distance d.sub.sp
is at least 1.0 .mu.m. In addition, sensor 12 may be made of a
chemically inert material, such as Pt or Au, to prevent the risk of
corrosion or oxidation of sensor 12 posed by positioning sensor 12
at medium confronting surface 13. Furthermore, damage due to
smearing of sensor 12 at medium confronting surface 13 may be
prevented or greatly reduced by embedding sensor 12 in embedding
material 72 (e.g., Ta), which is a material less susceptible to the
effects of exposure to the space between transducing head 10 and
magnetic medium 60. Recessing sensor 12 from medium confronting
surface 13 by a few nanometers may also prevent smearing of sensor
12. Embedding material 72 may alternatively cover sensor 12 at the
medium confronting surface such that sensor 12 is encased in
embedding material 72. Such measures have minimal affect on the
sensitivity of sensor 12 since the thermal conductivity of metals
is generally high.
The size of sensor 12 at medium confronting surface 13 may be
minimized within design and operability constraints to consume less
space within transducing head 10. A smaller sensor 12 also results
in increased sensitivity to changes in the HMS due to a higher
resistance across sensor 12 and a larger temperature variation for
the same energy accumulated or dissipated.
While sensor 12 is shown disposed adjacent to a trailing edge of
main pole tip 48 in FIGS. 1 and 2, it will be appreciated that
sensor 12 may be alternatively located proximate to medium
confronting surface 13 at other locations in transducing head 10.
For example, sensor 12 may be located proximate to the trailing
side of first return pole 30, the leading side of main pole 34, the
trailing side of second return pole 38, or the leading side of
second return pole 38. In addition, sensor 12 may be disposed
adjacent to reader 14. This flexibility in the location of sensor
12 is important in configurations of transducing head 10, such as
the trailing shield design described above, that include a device
component between first return pole 30 and main pole 34.
Sensor 12 as described is simple and cost-effective to fabricate
and, since the detection of changes in the resistance across sensor
12 is based on electrical measurement, the magnetic fields
generated by adjacent structures have a minimal effect on the
operation of sensor 12. Also, the response time of sensor 12 to
changes in HMS is very high, so variations in HMS can be detected
very quickly. Consequently, sensor 12 may be employed to not only
detect changes in HMS, but also to sense the presence of asperities
on magnetic medium 60, map the topography of magnetic medium 60,
and provide real-time control of the HMS by incorporating feedback
control of the HMS based on signals from sensor 12.
In order to show the effect of changes in HMS on sensor 12, FIG. 3
is a graph of the difference in off-medium and on-medium resistance
of sensor 12 as a function of an applied writer heater power. The
writer heater is thermally coupled to main pole tip 48 such that,
when different levels of power are applied to the writer heater,
variations in HMS occur due to changes in the contours of main pole
tip 48 (sometimes referred to as thermal tip protrusion). The
on-medium and off-medium resistance is measured to compensate for
any change in resistance induced by variations in ambient
conditions (e.g., ambient temperature). In the simulated device,
sensor 12 was made of gold. The applied writer heater power results
in an increase in the temperature of main pole tip 48, which
produces a decrease in the HMS of transducing head 10. As the HMS
decreases with increasing writer heater power, heat is transferred
more efficiently between sensor 12 and magnetic medium 60, and the
on-medium resistance of sensor 12 decreases relative to the
off-medium resistance, as shown in FIG. 3.
Sensor 12 may be calibrated to precisely determine the clearance of
transducing head 10. This may be done prior to use to determine an
initial clearance of transducing head 10, as well as in-situ when
conditions within the magnetic recording system change the HMS over
time. The clearance may be determined by increasing the applied
writer heater power until transducing head 10 contacts magnetic
medium 60 (i.e., separation d.sub.hms equals 0). FIG. 4 is a graph
showing the effect on sensor 12 when the writer heater power is
increased until contact is made between transducing head 10 and
magnetic medium 60. In particular, line 75 shows the peak-to-peak
voltage across sensor 12 as a function of the applied writer heater
power. To determine the clearance, the applied writer heater power
when transducing head 10 contacts magnetic medium 60 is noted. This
occurs when the voltage across sensor 12 begins to sharply increase
(about 125 mW in FIG. 4). The writer heater power is then reduced
from the noted heater power at the point of contact to set the
clearance of transducing head 10. When setting the clearance, the
time transducing head 10 is in contact with magnetic medium 60 is
minimized to prevent damage to transducing head 10. Sensor 12 has a
high signal-to-noise ratio response even when the time in contact
is very short and the level of contact interference is low.
The n.sup.th harmonic of a read back signal applied to sensor 12
changes as the HMS changes according to Wallace's loss equation:
V(y+.DELTA.y)=V(y)e.sup.-nk.DELTA.y (Equation 2), where k is the
spatial frequency of the applied signal and .DELTA.y is the change
in the HMS. For a signal with wavelength .lamda., the change in
HMS, which can be used to set the writer heater power and ensure
the clearance is set to the desired value, is given by:
.DELTA..times..times..lamda..times..times..DELTA..times..times..function.-
.times..times. ##EQU00002##
Various measures may be taken to improve the sensitivity of sensor
12 to changes in the HMS. For example, placing sensor 12 at or near
medium confronting surface 13 increases the response of sensor 12
to changes in the HMS. In addition, as indicated by Equation 1, the
detection sensitivity may be improved by increasing the temperature
between transducing head 10 and magnetic medium 60. In current
magnetic recording systems, the HMS is less than 100 .ANG., which
are dimensions that result in heat transfer between transducing
head 10 and magnetic medium 60 being dominated by ballistic
transfer. The temperature of transducing head 10 may be increased
by either increasing the amplitude of sense current I.sub.S
provided through sensor 12, or by adding an additional heat source
proximate to sensor 12. Also, sensitivity may be further increased
by removing heat sinks or positioning heat sinks further from
sensor 12 to ensure that heat is dissipated primarily through
magnetic medium 60.
FIGS. 5A-5D are schematic illustrations of configurations for
incorporating a sensor heater 80 with the sensor 12 to increase the
temperature (and thus, the sensitivity) of sensor 12. In the
embodiments shown, sensor 12 is disposed at medium confronting
surface 13 and sensor heater 80 is disposed adjacent to sensor 12
on a side opposite medium confronting surface 13. Sensor heater 80
is arranged relative to sensor 12 such that they are structurally
aligned substantially parallel to each other. In addition, sensor
heater 80 is separated from sensor 12 by an insulating material
(not shown). It will appreciated that while a single sensor heater
80 is shown in each of the circuits of FIGS. 5A-5D, a plurality of
heaters 80 may alternatively be connected in series with each
other.
As sensor 12 and sensor heater 80 are moved relative to magnetic
medium 60, sensor heater 80 dissipates thermal energy by conducting
through the insulating material, through sensor 12, through medium
confronting surface 13 into the space between transducing head 10
and magnetic medium 60, and finally into magnetic medium 60. The
current through sensor heater 80 is set such that sensor 12 has a
maximum temperature dependent resistance change at a normal
operating HMS.
FIG. 5A shows an embodiment of sensor heater 80 positioned relative
to sensor 12 in which sensor 12 receives sense current I.sub.S via
connection pads A and D, while sensor heater 80 is biased via
connection pads B and C. By providing biasing via separate
connection pads, sensor 12 and sensor heater 80 may be biased with
separate currents, which allows for independent control of the
biasing of these elements. In addition, there is no resistance
interaction component in the output signal from sensor 12 since
sensor heater 80 is provided on a separate biasing circuit.
In order to reduce the number of connection pads necessary for the
incorporation of sensor heater 80 with sensor 12, sensor 12 and
sensor heater 80 may share connections to the biasing current. For
example, FIG. 5B shows a configuration that includes three
connection pads A, B, and C (i.e., one additional pad compared to a
system without sensor heater 80). In this configuration, both
sensor 12 and sensor heater 80 are connected to connection pad A
(which may be a common or grounded node), while the other end of
sensor heater 80 is connected to connection pad B and the other end
of sensor 12 is connected to connection pad C. Similar to the
embodiment shown in FIG. 5A, sensor 12 and sensor heater 80 may be
biased with separate currents, which allows for independent control
of the biasing of these elements. In addition, there is no
resistance interaction component in the output signal from sensor
12 since sensor heater 80 is provided on a separate biasing
circuit. Furthermore, for very high frequency response signals, the
three-connection configuration provides the capability to route a
common or ground circuit near the connection wires to achieve
improved control of electrical signal transmission properties and
improved noise immunity in the presence of any environmental common
mode electrical interference.
Sensor 12 and sensor heater 80 may also be connected in
configurations that include two connection pads A and B, such as
the configurations shown in FIGS. 5C and 5D, to utilize the same
biasing current source for both sensor 12 and sensor heater 80. In
FIG. 5C, sensor 12 and sensor heater 80 are connected in parallel.
The magnitude of the biasing current that flows to each of sensor
12 and sensor heater 80 may be controlled by selecting resistance
values of each of the components. For example, if sensor heater 80
has a resistance of 50.OMEGA. and sensor 12 has a normal operating
temperature resistance of 200.OMEGA., 80% of the biasing current
flows though sensor heater 80 while 20% of the biasing current
flows through sensor 12. Consequently, the level of heating
provided by sensor heater 80 can be controlled by the relative
resistances of sensor 12 and sensor heater 80.
Capacitor 82 and inductor 84 are also shown in the circuit of FIG.
5C. Capacitor 82 is connected in series with sensor 12 and inductor
84 is connected in series with sensor heater 80. The capacitance of
capacitor 82 may be increased using materials with higher
dielectric values. With this configuration, sensor 12 may be
operated in high frequency mode (e.g., a high frequency carrier
current riding on a large DC current) and, since sense current
I.sub.S is much smaller than the current through sensor heater 80,
sensor heater 80 essentially operates in DC mode. In addition,
inductor 84 ensures that there is little to no AC leakage to sensor
heater 80. Furthermore, this configuration increases the life of
sensor 12 by reducing any electromigration risk that may arise if
the circuit is operated in DC mode. It should be noted that while a
single capacitor 82 and a single inductor 84 are shown, a plurality
of capacitors and/or inductors may alternatively be incorporated
into the circuit shown.
In FIG. 5D, sensor 12 and sensor heater 80 are connected in series.
The heat generated by sensor heater 80 is a function of the voltage
drop across sensor heater 80, so the level of heating may be
controlled by adjusting the resistance of sensor heater 80 and/or
the biasing current provided on connection pads A and B. In
alternative embodiments, a plurality of sensor heaters 80 may be
connected in series with sensor 12 and arranged in a stacked
configuration extending from medium confronting surface 13.
As described above, sensor 12 may be used in various applications
related to the HMS of transducing head 10. For example, the output
of sensor 12 may be monitored to detect the presence of asperities
and other irregularities protruding from the surface of magnetic
medium 60. When sensor 12 encounters or collides with an asperity
on magnetic medium 60, sensor 12 experiences a sharp increase or
spike in temperature resulting from frictional heating associated
with the contact forces between sensor 12 and the asperity. This
temperature spike results in a detectable change in the resistance
across sensor 12. After contact with the asperity, the temperature
of sensor 12 may be monitored as it recovers from the contact event
and reverts to the normal operating HMS. In this way, transducing
head 10 may return to normal operation (and rewrite or reread any
skipped or missed data caused by the contact event) after the HMS
returns to normal.
FIG. 6 shows the response of sensor 12 to contact with an asperity
on magnetic medium 60. In the device tested, sensor 12 was
comprised of gold, and sensor 12 came into contact with a 20 nm
laser bump formed on magnetic medium 60. Trace 90 shows the sharp
increase in voltage across sensor 12 resulting from the thermal
spike when sensor 12 contacted the laser bump. Sensor 12 returns to
its normal operating voltage after contacting the asperity in less
than 20 .mu.s. Consequently, the detection frequency of sensor 12
may be on the order of 1.0 MHz to assure detection of the voltage
variation caused by contact with the asperity.
Sensor 12 may also be employed to provide in-situ control of the
HMS based on detected thermal variations in sensor 12. FIG. 7 is a
block diagram of feedback control system 100 for adjusting the HMS
of transducing head 10 in response to changes in the HMS as
detected by sensor 12. Control system 100 includes controller 102
for communicating with sensor 12, reader 14, writer 16, and heater
80. Heater 80 generates heat in response to signals from controller
102 to raise the temperature of sensor 12 and main pole tip 48 of
writer 16. While a single heater 80 is shown in FIG. 7, separate
heaters for sensor 12 and writer 16 may alternatively be provided
in control system 100. In addition, each heater may consist of a
single or a plurality of heater elements, and may have the
configurations described with regard to FIGS. 5A-5D.
In an alternative embodiment, sensor 12 may be connected in
parallel with writer 16 such that the same current is applied to
sensor 12 and writer 16 during operation. A low pass filter in
series with sensor 12 ensures that sensor 12 is operated at a much
lower frequency than writer 16. This allows the response of sensor
12 to remain detectably separate from the response of writer
16.
In operation, controller 102 controls operation of reader 14 and
writer 16 to read information from and write information to
magnetic medium 60. Controller 102 also measures the resistance
across sensor 12 to monitor thermal variations caused by changes in
HMS. As described above, controller 102 may also increase the
sensitivity of sensor 12 by heating sensor 12 with sensor heater
80. Controller 102 may compare the measured resistance across
sensor 12 to a stored resistance related to the normal HMS to
determine whether the d.sub.hms has increased or decreased. Based
on this determination, controller 102 controls transducing head 10
to adjust separation d.sub.hms back to the normal HMS. In control
system 100, controller 102 may accomplish this by operating heater
80 to heat writer 16 (and in particular main pole tip 48). The
change in temperature causes the contours of main pole tip 48 to
change at medium confronting surface, resulting in a change in the
HMS. Since the level of heating of writer 16 may be controlled to
produce the desired level of change in the HMS, and because the
response time of sensor 12 to changes in the HMS is fast,
controller 102 can adjust the HMS to the normal HMS very
quickly.
In addition to being able to dynamically control the HMS of
transducing head 10, the real-time detection of the HMS with sensor
12 has other applications. For example, controller 102 may monitor
the thermal variations in sensor 12 to generate a map of the
topography of magnetic medium 60. Thus, because sensor 12 is
sensitive to irregularities on magnetic medium 60 at the nanometer
level, control system 100 may be used to screen out media with a
large number of nano-asperities during media quality certification.
In addition, in magnetic recording systems having an air bearing
surface, sensor 12 may be employed as a tool to assess the
functionality of various air bearing designs by detecting the level
of air bearing modulation for each of the designs. Furthermore,
lube puddles and other irregularities on magnetic medium 60 may
result in an increase in separation d.sub.hms (and a corresponding
change in the resistance across sensor 12) when transducing head 10
passes over these irregularities, which may lead to poor
readability or writability and drive failures in these areas. When
this occurs, controller 102 may return to the portion of magnetic
medium 60 corresponding to the increase in HMS to reread or rewrite
the skipped data. In order to compensate for changes in HMS at the
location of the irregularity, controller 102 may activate heater 80
to adjust separation d.sub.hms to maintain a constant HMS.
In summary, the present invention relates to a system including a
magnetic device for writing to and reading from a magnetic medium
and a sensor disposed-adjacent to the magnetic device and proximate
to the magnetic medium. The sensor generates signals related to
thermal variations in the sensor caused by changes in a distance
between the magnetic device and the magnetic medium. By having the
sensor as a separate element from the magnetic device, the
effectiveness and lifespan of the magnetic device is improved. In
addition, the signal generated by the sensor in response to changes
in the distance between the magnetic device and the magnetic medium
is separate from the signals produced by the magnetic device,
making the sensor signals easier to detect and measure.
Furthermore, the sensor can provide signals related to the distance
between the magnetic device and magnetic medium in-situ, allowing
for adjustments to this distance to be made quickly in response to
variations in the distance.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention. For example, while sensor 12
has been described as a single layer of chemically inert material,
sensor 12 may alternatively be implemented as a thermocouple
junction including two wires made up of two dissimilar metals used
as a thermal sensor based on the Seebeck effect.
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